Melt the butter over low heat.
Stir in the flour.
Cook 1 minute.
Add a little cayenne pepper, salt, and mustard to taste.
Slowly add the milk, stirring constantly.
Cook until smooth and thick.
Add the cheese.
Stir until it is melted.
Remove from the heat.
Beat the egg yolks.
Stir them into the cheese sauce.
Beat until well blended.
Cool until lukewarm or cooler.
Beat the egg whites until stiff.
Mix a large spoonful into the sauce.
Now fold the sauce into the remaining whites.
Pour gently into a buttered 1 1/2 quart souffle dish.
Bake 20 to 25 minutes at 350Â°.